Added ability to load libraries from an external source

Added loadnExternalLibrary method to the import process which
will check an optional Sk.externalLibraries config for info on
where to find the library
Merge remote-tracking branch 'upstream/master'
Preserve cell vars across yields.
Add test case for cell variables/yield interactions.
Merge pull request #240 from rixner/master

Preserve cell variables across calls to yield.
Merge pull request #239 from waywaaard/precision_format

Fixes firefox specific bug for precision parising in str.js
Merge pull request #237 from trinketapp/master

Added ability to load libraries from an external source dynamically at compile time
pre- rename conflicting test files
Merge branch 'pr' of github.com:waywaaard/skulpt into waywaaard-pr
Merge branch 'master' of github.com:skulpt/skulpt
Enable min/max to handle arbitrary iterables.
Typecheck other parameter in sq$concat.
Handle error cases in min()/max().
min()/max() tests.
Merge remote-tracking branch 'upstream/master'
Merge pull request #242 from rixner/master

Minor fixes to min()/max() and tuples
Handle list += properly.
Test case for list +=.
Merge pull request #244 from rixner/master

Make list += behave correctly.
Merge remote-tracking branch 'upstream/master'